1. Field of the Invention
The present invention relates to a belt conveyance baking furnace suitable for heat treatment of a small workpiece to be baked.
2. Description of the Related Art
Heretofore, when baking small ceramic parts such as ceramic capacitors or firing external electrodes, a mesh belt type continuous baking furnace as shown in FIGS. 1 to 3 has been employed. This baking furnace includes a furnace body 1 associated with an entry section 1a, a heating section 1b, a cooling section 1c, and a delivery section 1d. A tubular muffle 2 is disposed inside the furnace body 1 to extend over both the heating section 1b and the cooling section 1c. A mesh belt 3 extends through the muffle 2 and is driven to run by a drive roller 4 so that a workpiece on the belt 3 is heat-treated while it is continuously conveyed with the belt 3. Additionally, denoted by 5 is a motor, 6 are table rollers, and 7 are tension rollers.
In some cases, the mesh belt 3 directly contacts the bottom of the muffle 2. In other cases, rails made of metal or quartz are laid on the bottom of the muffle 2 to support the mesh belt 3 on the rails.
In the belt conveyance baking furnace as mentioned above, however, since the mesh belt 3 is driven to run while contacting the muffle 2 or the rails, the mesh belt 3 is subjected to friction resistance (usually the coefficient of friction f=0.3-0.5) and hence undergoes a large tension. Therefore, the mesh belt 3 must have the strength endurable against such a large tension, which has been disadvantageous in increasing the weight and size of the belt. Also, the large belt weight increases heat capacity of the member moving through the furnace. It has thus been difficult to raise and lower the furnace temperature to desired levels in a short time (e.g., within 20 minutes).
FIG. 4 shows one example of a temperature profile of the baking furnace shown in FIG. 1. As is apparent from FIG. 4, because of the mesh belt 3 having large heat capacity, a characteristic curve in the temperature raising process is so gentle that it takes about 20 minutes to raise the furnace temperature by 900.degree. C. Similarly, in the temperature lowering process, it takes about 30 minutes to lower the furnace temperature by 700.degree. C. Accordingly, there have been problems that the baking needs an inconvenient amount of time and the treatment efficiency is poor.